usbfs: don't store bad pointers in registration

This patch (as1107) fixes a small bug in the usbfs registration and
unregistration code.  It avoids leaving an error value stored in the
device's usb_classdev field and it avoids trying to unregister a NULL
pointer.  (It also fixes a rather extreme overuse of whitespace.)

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>